An essential role for Mad homology domain 1 in the association of Smad3 with histone deacetylase activity*

Down-regulation of osteopontin mediates a novel mechanism underlying the cytostatic activity of TGF-β

PURPOSE

Loss of a cytostatic response to TGF-β has been implicated in multiple hyper-proliferative disorders, including cancer. Although several key genes involved in the cytostatic activity of TGF-β have in the past been identified, its exact mode of action is yet to be elucidated. A comprehensive understanding of the mechanisms underlying the cytostatic activity of TGF-β may open up new avenues for the development of therapeutic strategies.

METHODS

Quantitative real-time RT-PCR was used to assess osteopontin (OPN) gene expression in human hepatoma-derived Huh-7 and lung adenocarcinoma-derived A549 cells. Reporter assays using an OPN promoter-luciferase construct and its mutated counterparts were performed to assess its transcriptional activity. Binding of Smad4 to the OPN gene promoter was investigated using chromatin immunoprecipitation (CHIP). The putative role of Smad4 in OPN gene expression down-regulation was also assessed using a shRNA-mediated knockdown strategy. The anti-proliferative effect of TGF-β on different cancer-derived cell lines was determined using the cell proliferation reagent WST-1.

RESULTS

We found that the OPN expression levels dose-dependently decreased in TGF-β-treated Huh-7 and A549 cells. Our reporter assays indicated that this TGF-β-induced repression occurred at the transcriptional level, and could largely be abrogated by disruption of an element (TIE2) similar to the TGF-β inhibitory element found in other TGF-β-repressed genes. Our CHIP assay revealed that the Smad protein complex specifically binds to the OPN gene promoter, and that the TGF-β-mediated inhibition of OPN was lost upon shRNA-mediated knockdown of Smad4. Moreover, we found that the deregulation of OPN gene expression by TGF-β occurred concomitantly with loss of the TGF-β anti-proliferative response, whereas a neutralizing anti-OPN antibody partially restored this response.

CONCLUSIONS

Our results indicate that the OPN gene is a direct target of Smad-mediated TGF-β signaling, implying that OPN expression inhibition serves as a novel mechanism underlying the cytostatic activity of TGF-β.

By the company they keep: interaction networks define the binding ability of transcription factors

Access to genome-wide data provides the opportunity to address questions concerning the ability of transcription factors (TFs) to assemble in distinct macromolecular complexes. Here, we introduce the PAnDA (Protein And DNA Associations) approach to characterize DNA associations with human TFs using expression profiles, protein–protein interactions and recognition motifs. Our method predicts TF binding events with >0.80 accuracy revealing cell-specific regulatory patterns that can be exploited for future investigations. Even when the precise DNA-binding motifs of a specific TF are not available, the information derived from protein-protein networks is sufficient to perform high-confidence predictions (area under the ROC curve of 0.89). PAnDA is freely available at http://service.tartaglialab.com/new_submission/panda.

Molecular mechanism of TGF-β signaling pathway in colon carcinogenesis and status of curcumin as chemopreventive strategy

Colon cancer is one of the third most common cancer in man, the second most common cancer in women worldwide, and the second leading cause of mortality in the USA. There are a number of molecular pathways that have been implicated in colon carcinogenesis, including TGF-β/Smad signaling pathway. TGF-β (transforming growth factor-beta) signaling pathway has the potential to regulate various biological processes including cell growth, differentiation, apoptosis, extracellular matrix modeling, and immune response. TGF-β signaling pathway acts as a tumor suppressor, but alterations in TGF-β signaling pathway promotes colon cancer cell growth, migration, invasion, angiogenesis, and metastasis. Here we review the role of TGF-β signaling cascade in colon carcinogenesis and multiple molecular targets of curcumin in colon carcinogenesis. Elucidation of the molecular mechanism of curcumin on TGF-β signaling pathway-induced colon carcinogenesis may ultimately lead to novel and more effective treatments for colon cancer.

microRNA are Central Players in Anti- and Profibrotic Gene Regulation during Liver Fibrosis

MicroRNA (miRNA) are small non-coding RNA molecules that posttranscriptionally effect mRNA stability and translation by targeting the 3'-untranslated region (3'-UTR) of various transcripts. Thus, dysregulation of miRNA affects a wide range of cellular processes such as cell proliferation and differentiation involved in organ remodeling processes. Divergent miRNA patterns were observed during chronic liver diseases of various etiologies. Chronic liver diseases result in uncontrolled scar formation ending up in liver fibrosis or even cirrhosis. Since it has been shown that miR-29 dysregulation is involved in synthesis of extracellular matrix proteins, miR-29 is of special interest. The importance of miR-29 in hepatic collagen homeostasis is underlined by in vivo data showing that experimental severe fibrosis is associated with a prominent miR-29 decrease. The loss of miR-29 is due to the response of hepatic stellate cells to exposure to the profibrogenic mediators TGF-β and PDGF-BB. Several putative binding sites for the Smad proteins and the Ap1 complex are located in the miR-29 promoter, which are suggested to mediate miR-29 decrease in fibrosis. Other miRNA are highly increased after profibrogenic stimulation, such as miR-21. miR-21 is transcriptionally upregulated in response to Smad-3 rather than Smad-2 activation after TGF-β stimulation. In addition, TGF-β promotes miR-21 expression by formation of a microprocessor complex containing Smad proteins. Elevated miR-21 may then act as a profibrogenic miRNA by its repression of the TGF-β inhibitory Smad-7 protein.

Regulation of cell cycle and DNA repair in post-mitotic GABA neurons in psychotic disorders

Disturbances of cell cycle regulation and DNA repair in post-mitotic neurons have been implicated in degenerative and malignant diseases of the human brain. Recent work is now suggesting that abnormal regulation of these functions in GABA cells of the adult hippocampus may also play a role in two neuropsychiatric disorders. In schizophrenia and bipolar disorder, a network of genes involved in the regulation of GAD₆₇, a marker for the functional differentiation of GABA cells, show pronounced changes in expression and include kainate receptor subunits, TGFβ and Wnt signaling pathways, epigenetic factors and transcription factors. One of these genes, cyclin D2, is involved in the regulation of cell cycle and DNA repair and appears to be a pivotal element in linking GAD₆₇ expression with these functional clusters of genes. Dysfunction of post-mitotic GABAergic neurons in the adult hippocampus of patients with psychotic disorders is associated with changes in the expression of genes that are involved in the maintenance of functional and genomic integrity of GABA cells. The nature of these changes is quite different in schizophrenia and bipolar disorder, suggesting that a common cell phenotype (in this case, decreased GAD₆₇ expression) may involve two fundamentally different molecular endophenotypes and reflect unique susceptibility genes involved in the respective disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Essential role of TGF-beta signaling in glucose-induced cell hypertrophy

In multicellular organisms, cell size is tightly controlled by nutrients and growth factors. Increasing ambient glucose induces enhanced protein synthesis and cell size. Continued exposure of cells to high glucose in vivo, as apparent under pathological conditions, results in cell hypertrophy and tissue damage. We demonstrate that activation of TGF-beta signaling has a central role in glucose-induced cell hypertrophy in fibroblasts and epithelial cells. Blocking the kinase activity of the TbetaRI receptor or loss of its expression prevented the effects of high glucose on protein synthesis and cell size. Exposure of cells to high glucose induced a rapid increase in cell surface levels of the TbetaRI and TbetaRII receptors and a rapid activation of TGF-beta ligand by matrix metalloproteinases, including MMP-2 and MMP-9. The consequent autocrine TGF-beta signaling in response to glucose led to Akt-TOR pathway activation. Accordingly, preventing MMP-2/MMP-9 or TGF-beta-induced TOR activation inhibited high glucose-induced cell hypertrophy.

Ligand-dependent ubiquitination of Smad3 is regulated by casein kinase 1 gamma 2, an inhibitor of TGF-beta signaling

Transforming growth factor-beta (TGF-beta) elicits a variety of cellular activities primarily through a signaling cascade mediated by two key transcription factors, Smad2 and Smad3. Numerous regulatory mechanisms exist to control the activity of Smad3, thereby modulating the strength and specificity of TGF-beta responses. In search for potential regulators of Smad3 through a yeast two-hybrid screen, we identified casein kinase 1 gamma 2 (CKIgamma2) as a novel Smad3-interacting protein. In mammalian cells, CKIgamma2 selectively and constitutively binds Smad3 but not Smad1, -2 or -4. Functionally, CKIgamma2 inhibits Smad3-mediated TGF-beta responses including induction of target genes and cell growth arrest, and this inhibition is dependent on CKIgamma2 kinase activity. Mechanistically, CKIgamma2 does not affect the basal levels of Smad proteins or activity of the receptors. Rather, CKIgamma2 preferentially promotes the ubiquitination and degradation of activated Smad3 through direct phosphorylation of its MH2 domain at Ser418. Importantly, mutation of Ser418 to alanine or aspartic acid causes an increase or decrease of Smad3 activity, respectively, in the presence of TGF-beta. CKIgamma2 is the first kinase known to mark activated Smad3 for destruction. Given its negative function in TGF-beta signaling and its reported overexpression in human cancers, CKIgamma2 may act as an oncoprotein during tumorigenesis.

Doxorubicin inhibits TGF-beta signaling in human lung carcinoma A549 cells

Doxorubicin is a DNA-damaging drug, commonly used for treatment of cancer patients. Doxorubicin causes not only cytotoxic and cytostatic effects, but also inhibits metastasis formation, while TGFbeta1 (Transforming Growth Factor-beta1) is a cytokine that is often up-regulated in human cancers and can promote metastasis formation. We have studied the influence of Doxorubicin on TGFbeta signaling in tumor cells. Here we have demonstrated that Doxorubicin inhibited TGFbeta-signaling in human lung adenocarcinoma A549 cells, namely, it blocked TGFbeta1-induced activation of Smad3-responsive CAGA(12)-Luc reporter, but did not affect c-myc-Luc reporter. That effect was observed as early as after 1-3 h of treating these cells with Doxorubicin, while the other drugs cisplatin or methotrexate did not alter activation of CAGA(12)-Luc reporter under the same conditions. Besides, after 1 h action, Doxorubicin abrogated TGFbeta-induced translocation of Smad3-protein from the cytoplasm to the nucleus. Down-regulation of expression of Smad2, Smad3, and Smad4 proteins, and up-regulation of inhibitory Smad7 protein upon Doxorubicin treatment, were found after 12-24 h of Doxorubicin treatment. Phosphorylation of Smad2/3 proteins was also affected by Doxorubicin. Summarizing, we have found that human tumor cells treatment with Doxorubicin resulted in the inhibition of TGFbeta-signaling at both early (1 h) and later (12 h) stages of the drug action. Such inhibition can be a new potential mechanism for Doxorubicin action towards tumor cells.

A novel role of transforming growth factor beta1 in transcriptional repression of human cholesterol 7alpha-hydroxylase gene

BACKGROUND & AIMS

Inhibition of cholesterol 7alpha-hydroxylase (CYP7A1) by bile acids and inflammatory cytokines provides an important mechanism to protect hepatocytes from bile acid toxicity during cholestasis. Transforming growth factor beta1 (TGFbeta1) released by hepatic stellate cells during chronic liver injury plays a critical role in liver inflammation and fibrogenesis. The objective of this study is to investigate the role of TGFbeta1 in hepatic bile acid synthesis.

METHODS

mRNA expressions in primary human hepatocytes and HepG2 cells were measured by quantitative real-time polymerase chain reaction. Reporter assay, glutathione-S-transferase pull-down assay, adenovirus-mediated gene transduction, and chromatin immunoprecipitation assay were used to study the mechanism of TGFbeta1 regulation of CYP7A1 gene transcription.

RESULTS

TGFbeta1 inhibited the mRNA expression of CYP7A1 and bile acid synthesis in HepG2 cells and primary human hepatocytes. Mothers against decapentaplegic homolog (Smad3) inhibited both CYP7A1 promoter activity and mRNA expression by inhibiting DNA-binding activity of hepatocyte nuclear factor 4alpha (HNF4alpha). The histone deacetylase (HDAC) inhibitor Tricostatin A partially blocked the TGFbeta1 inhibition of CYP7A1 mRNA expression, whereas TGFbeta1 decreased histone 3 acetylation in the CYP7A1 chromatin. TGFbeta1 treatment and adenovirus Smad3 reduced HNF4alpha binding but increased the recruitment of Smad3, HDAC1, and a repressor mSin3A to the CYP7A1 chromatin.

CONCLUSIONS

This study provides the first evidence that TGFbeta1 represses CYP7A1 gene transcription in human hepatocytes by a mechanism involving Smad3-dependent inhibition of HNF4alpha and HDAC remodeling of CYP7A1 chromatin. The TGFbeta1/Smad3 signaling may reduce bile acid synthesis in the liver and prevent hepatocyte injury in cholestatic liver disease.

A tale of two proteins: differential roles and regulation of Smad2 and Smad3 in TGF-beta signaling

Transforming growth factor-beta (TGF-beta) is an important growth inhibitor of epithelial cells, and insensitivity to this cytokine results in uncontrolled cell proliferation and can contribute to tumorigenesis. Smad2 and Smad3 are direct mediators of TGF-beta signaling, however little is known about the selective activation of Smad2 versus Smad3. The Smad2 and Smad3 knockout mouse phenotypes and studies comparing Smad2 and Smad3 activation of TGF-beta target genes, suggest that Smad2 and Smad3 have distinct roles in TGF-beta signaling. The observation that TGF-beta inhibits proliferation of Smad3-null mammary gland epithelial cells, whereas Smad3 deficient fibroblasts are only partially growth inhibited, suggests that Smad3 has a different role in epithelial cells and fibroblasts. Herein, the current understanding of Smad2 and Smad3-mediated TGF-beta signaling and their relative roles are discussed, in addition to potential mechanisms for the selective activation of Smad2 versus Smad3. Since alterations in the TGF-beta signaling pathway play an important role in promoting tumorigenesis and cancer progression, methods for therapeutic targeting of the TGF-beta signaling pathway are being pursued. Determining how Smad2 or Smad3 differentially regulate the TGF-beta response may translate into developing more effective strategies for cancer therapy.

Acetylation of Smad2 by the co-activator p300 regulates activin and transforming growth factor beta response

Transforming growth factor beta (TGFbeta) signals primarily through the Smad proteins to regulate cell growth, differentiation, and extracellular matrix production. Post-translational modifications, such as phosphorylation, play an important role in the regulation of the Smad proteins. TGFbeta signaling results in the phosphorylation of Smad2 and Smad3 that then oligomerize with Smad4 and translocate into the nucleus to initiate transcription of TGFbeta target genes. The initiation of transcription is significantly enhanced by the direct interaction of the Smad complex with p300/CBP (CREB-binding protein), a co-activator with intrinsic acetyltransferase activity. However, how p300/CBP enhances transcription through this interaction is not entirely understood. In this report, we show that Smad2, but not the highly homologous Smad3, can be acetylated by p300/CBP in a ligand-dependent manner. At least three lysine residues, Lys(19), Lys(20), and Lys(39), are required for efficient acetylation of Smad2, as mutations altering these lysines abolished Smad2 acetylation in vivo. This acetylation event is required for the ability of Smad2 to mediate activin and TGFbeta signaling. Mutation of the three key lysine residues did not alter the stability of Smad2 or the ability of Smad2 to form a complex with Smad4 on promoter DNA, but it prevented nuclear accumulation of Smad2 and subsequent TGFbeta and activin responses. Thus, our studies reveal a novel mechanism of modulating Smad2 activity and localization through protein acetylation.

Function of the two Xenopus smad4s in early frog development

Signals from the transforming growth factor beta family members are transmitted in the cell through specific receptor-activated Smads and a common partner Smad4. Two Smad4 genes (alpha and beta/10, or smad4 and smad4.2) have been isolated from Xenopus, and conflicting data are reported for Smad4beta/10 actions in mesodermal and neural induction. To further understand the functions of the Smad4s in early frog development, we analyzed their activities in detail. We report that Smad10 is a mutant form of Smad4beta that harbors a missense mutation of a conserved arginine to histidine in the MH1 domain. The mutation results in enhanced association of Smad10 with the nuclear transcription corepressor Ski and leads to its neural inducing activity through inhibition of bone morphogenetic protein (BMP) signaling. In contrast to Smad10, both Smad4alpha and Smad4beta enhanced BMP signals in ectodermal explants. Using antisense morpholino oligonucleotides (MOs) to knockdown endogenous Smad4 protein levels, we discovered that Smad4beta was required for both activin- and BMP-mediated mesodermal induction in animal caps, whereas Smad4alpha affected only the BMP signals. Neither Smad4 was involved directly in neural induction. Expression of Smad4beta-MO in early frog embryos resulted in reduction of mesodermal markers and defects in axial structures, which were rescued by either Smad4alpha or Smad4beta. Smad4alpha-MO induced only minor deficiency at late stages. As Smad4beta, but not Smad4alpha, is expressed at high levels maternally and during early gastrulation, our data suggest that although Smad4alpha and Smad4beta may have similar activities, they are differentially utilized during frog embryogenesis, with only Smad4beta being essential for mesoderm induction.

SPECIFICITY AND VERSATILITY IN TGF-β SIGNALING THROUGH SMADS

The TGF-beta family comprises many structurally related differentiation factors that act through a heteromeric receptor complex at the cell surface and an intracellular signal transducing Smad complex. The receptor complex consists of two type II and two type I transmembrane serine/threonine kinases. Upon phosphorylation by the receptors, Smad complexes translocate into the nucleus, where they cooperate with sequence-specific transcription factors to regulate gene expression. The vertebrate genome encodes many ligands, fewer type II and type I receptors, and only a few Smads. In contrast to the perceived simplicity of the signal transduction mechanism with few Smads, the cellular responses to TGF-beta ligands are complex and context dependent. This raises the question of how the specificity of the ligand-induced signaling is achieved. We review the molecular basis for the specificity and versatility of signaling by the many ligands through this conceptually simple signal transduction mechanism.

Repression of Runx2 function by TGF-beta through recruitment of class II histone deacetylases by Smad3

Transforming growth factor-beta (TGF-beta) inhibits osteoblast differentiation through inhibition of the function of Runx2 (Cbfa1) by Smad3. The mechanism through which TGF-beta/Smad3 inhibits Runx2 function has not been characterized. We show that TGF-beta induces histone deacetylation, primarily of histone H4, at the osteocalcin promoter, which is repressed by TGF-beta, and that histone deacetylation is required for repression of Runx2 by TGF-beta. This repression occurs through the action of the class IIa histone deacetylases (HDAC)4 and 5, which are recruited through interaction with Smad3 to the Smad3/Runx2 complex at the Runx2-binding DNA sequence. Accordingly, HDAC4 or 5 is required for efficient TGF-beta-mediated inhibition of Runx2 function and is involved in osteoblast differentiation. Our results indicate that class IIa HDACs act as corepressors for TGF-beta/Smad3-mediated transcriptional repression of Runx2 function in differentiating osteoblasts and are cell-intrinsic regulators of osteoblast differentiation.

Endogenous Inhibitors of Nuclear Factor-κB, An Opportunity for Cancer Control: Fig. 1

Excessive and prolonged activation of nuclear factor-kappaB (NF-kappaB) has been linked to numerous human diseases, especially cancer, because of the elevated expression of genes encoding antiapoptotic proteins, cytokines, chemokines, cell adhesion molecules, and so on. Eukaryotic cells have developed multiple mechanisms to keep this ubiquitous transcription factor in check. In addition to the inhibitor of kappaB family proteins, a number of endogenous molecules that negatively regulate the activation or activity of NF-kappaB have been identified. These molecules include A20, CYLD, cyPG15-deoxy-Delta(12,14)-prostaglandin J(2), Foxj1, Twist proteins, and beta-arrestins. The extended list of these endogenous inhibitors of NF-kappaB may provide new opportunities for the development of novel strategies for the intervention of malignant transformation. The question to be asked is how NF-kappaB is sustained activated in a number of cancers in which so many antagonists are surrounded.

Suv39h histone methyltransferases interact with Smads and cooperate in BMP-induced repression

Smad proteins transduce signals from transforming growth factor-beta (TGF-beta) superfamily ligands to regulate the expression of target genes. In order to identify novel partners of Smad proteins in transcriptional regulation, we performed a two-hybrid screen using Smad5, a protein that is activated predominantly by bone morphogenetic protein (BMP) signaling. We identified an interaction between Smad5 and suppressor of variegation 3-9 homolog 2 (Suv39h2), a chromatin modifier enzyme. Suv39h proteins are histone methyltransferases that methylate histone H3 on lysine 9, resulting in transcriptional repression or silencing of target genes. Biochemical studies in mammalian cells demonstrated that Smad5 binds to both known mammalian isoforms of Suv39h proteins, and that Smad proteins activated by the TGF-beta signaling pathway, Smad2 and Smad3, do not bind with significant affinity. Functional studies using the muscle creatine kinase (MCK) promoter, which is suppressed by BMP signaling, demonstrate that Suv39h proteins and Smads cooperate to repress promoter activity. These data suggest a model where association of Smad proteins with Suv39h methyltransferases can repress or silence genes involved in developmental processes, and argues that inefficient gene repression may result in the alteration of the differentiated phenotype. Thus, examination of the Smad-Suv interaction may provide insight into the mechanism of phenotypic determination mediated by BMP signaling.

Menin is required for bone morphogenetic protein 2- and transforming growth factor beta-regulated osteoblastic differentiation through interaction with Smads and Runx2

Menin, the product of the multiple endocrine neoplasia type 1 (MEN1) gene, is required for commitment of multipotential mesenchymal stem cells to the osteoblast lineage, however, it inhibits their later differentiation (Sowa, H., Kaji, H., Canaff, L., Hendy, G.N., Tsukamoto, T., Yamaguchi, T., Miyazono, K., Sugimoto, T., and Chihara, K. (2003) J. Biol. Chem. 278, 21058-21069). Here, we have examined the mechanism of action of menin in regulating osteoblast differentiation using the mouse bone marrow stromal ST2 and osteoblast MC3T3-E1 cell lines. In ST2 cells, reduced menin expression achieved by transfection of menin antisense DNA (AS) antagonized bone morphogenetic protein (BMP)-2-induced alkaline phosphatase activity and osteocalcin and Runx2 mRNA expression. Menin was co-immunoprecipitated with Smad1/5 in ST2 and MC3T3-E1 cells, and inactivation of menin antagonized BMP-2-induced transcriptional activity of Smad1/5 in ST2 cells, but not MC3T3-E1 cells. Menin was co-immunoprecipitated with the key osteoblast regulator, Runx2, and AS antagonized Runx2 transcriptional activity and the ability of Runx2 to stimulate alkaline phosphatase activity only in ST2 cells but not in MC3T3-E1 cells. In the osteoblast MC3T3-E1 cells, transforming growth factor-beta and its signaling molecule, Smad3, negatively regulated Runx2 transcriptional activity. Menin and Smad3 were co-immunoprecipitated, and combined menin and Smad3 overexpression antagonized, whereas menin and the dominant-negative Smad3DeltaC together enhanced BMP-2-induced transcriptional activity of Smad1/5 and Runx2. Smad3 alone had no effect. Therefore, menin interacts physically and functionally with Runx2 in uncommitted mesenchymal stem cells, but not in well differentiated osteoblasts. In osteoblasts the interaction of menin and the transforming growth factor-beta/Smad3 pathway negatively regulates the BMP-2/Smad1/5- and Runx2-induced transcriptional activities leading to inhibition of late-stage differentiation.

Casein kinase Iepsilon plays a functional role in the transforming growth factor-beta signaling pathway

The transforming growth factor-beta (TGF-beta) signaling pathway is known to be involved in a wide range of biological events, including development, cellular differentiation, apoptosis, and oncogenesis. The TGF-beta signal is mediated by ligand binding to the type II receptor, leading to the recruitment and activation of the type I receptor, and subsequent activation of a family of intracellular signal transducing proteins called Smads. Here we report a regulatory role for casein kinase Iepsilon (CKIepsilon) in the TGF-beta signaling cascade. We find that CKIepsilon binds to all Smads and the cytoplasmic domains of the type I and type II receptors both in vitro and in vivo. The interaction of CKIepsilon with the type I and type II receptors is independent of TGF-beta stimulation, whereas the CKIepsilon/Smad interaction is transiently disrupted by ligand treatment. Additionally, CKIepsilon is able to phosphorylate the receptor-activated Smads (Smads 1-3 and 5) and the type II receptor in vitro. Transcriptional reporter assays reveal that transient overexpression of wild type CKIepsilon dramatically reduces basal reporter activity but enhances TGF-beta-stimulated transcription. Furthermore, overexpression of a kinase-dead mutant of CKIepsilon inhibits both basal and ligand-induced transcription, whereas inhibition of endogenous CKI catalytic activity with IC261 blocks only TGF-beta-stimulated reporter activity. Finally, knocking down CKIepsilon protein levels results in a significant increase in basal and TGF-beta-induced transcription. These results suggest that CKIepsilon plays a ligand-dependent, differential, and dual regulatory role within the TGF-beta signaling pathway.

Transforming growth factor beta-mediated transcriptional repression of c-myc is dependent on direct binding of Smad3 to a novel repressive Smad binding element

Smad proteins are the most well-characterized intracellular effectors of the transforming growth factor beta (TGF-beta) signal. The ability of the Smads to act as transcriptional activators via TGF-beta-induced recruitment to Smad binding elements (SBE) within the promoters of TGF-beta target genes has been firmly established. However, the elucidation of the molecular mechanisms involved in TGF-beta-mediated transcriptional repression are only recently being uncovered. The proto-oncogene c-myc is repressed by TGF-beta, and this repression is required for the manifestation of the TGF-beta cytostatic program in specific cell types. We have shown that Smad3 is required for both TGF-beta-induced repression of c-myc and subsequent growth arrest in keratinocytes. The transcriptional repression of c-myc is dependent on direct Smad3 binding to a novel Smad binding site, termed a repressive Smad binding element (RSBE), within the TGF-beta inhibitory element (TIE) of the c-myc promoter. The c-myc TIE is a composite element, comprised of an overlapping RSBE and a consensus E2F site, that is capable of binding at least Smad3, Smad4, E2F-4, and p107. The RSBE is distinct from the previously defined SBE and may partially dictate, in conjunction with the promoter context of the overlapping E2F site, whether the Smad3-containing complex actively represses, as opposed to transactivates, the c-myc promoter.

Smad6 recruits transcription corepressor CtBP to repress bone morphogenetic protein-induced transcription

Smad6 and Smad7 are inhibitory Smads induced by transforming growth factor beta-Smad signal transduction pathways in a negative-feedback mechanism. Previously it has been thought that inhibitory Smads bind to the type I receptor and block the phosphorylation of receptor-activated Smads, thereby inhibiting the initiation of Smad signaling. Conversely, few studies have suggested the possible nuclear functions of inhibitory Smads. Here, we present compelling evidence demonstrating that Smad6 repressed bone morphogenetic protein-induced Id1 transcription through recruiting transcriptional corepressor C-terminal binding protein (CtBP). A consensus CtBP-binding motif, PLDLS, was identified in the linker region of Smad6. Our findings show that mutation in the motif abolished the Smad6 binding to CtBP and subsequently its repressor activity of transcription. We conclude that the nuclear functions and physical interaction of Smad6 and CtBP provide a novel mechanism for the transcriptional regulation by inhibitory Smads.

Smad-dependent and Smad-independent pathways in TGF-beta family signalling

Transforming growth factor-beta (TGF-beta) proteins regulate cell function, and have key roles in development and carcinogenesis. The intracellular effectors of TGF-beta signalling, the Smad proteins, are activated by receptors and translocate into the nucleus, where they regulate transcription. Although this pathway is inherently simple, combinatorial interactions in the heteromeric receptor and Smad complexes, receptor-interacting and Smad-interacting proteins, and cooperation with sequence-specific transcription factors allow substantial versatility and diversification of TGF-beta family responses. Other signalling pathways further regulate Smad activation and function. In addition, TGF-beta receptors activate Smad-independent pathways that not only regulate Smad signalling, but also allow Smad-independent TGF-beta responses.

Activation of TGF-beta-Smad signaling pathway following polyamine depletion in intestinal epithelial cells

Smad proteins are transcription activators that are critical for transmitting transforming growth factor-beta (TGF-beta) superfamily signals from the cell surface receptors to the nucleus. Our previous studies have shown that cellular polyamines are essential for normal intestinal mucosal growth and that a decreased level of polyamines inhibits intestinal epithelial cell proliferation, at least partially, by increasing expression of TGF-beta/TGF-beta receptors. The current study went further to determine the possibility that Smads are the downstream intracellular effectors of activated TGF-beta/TGF-beta receptor signaling following polyamine depletion. Studies were conducted in IEC-6 cells derived from rat small intestinal crypts. Depletion of cellular polyamines by alpha-difluoromethylornithine (DFMO) increased basal levels of Smad3 and Smad4 proteins, induced their nuclear translocation, and stimulated Smad sequence-specific DNA-binding activity. Polyamine depletion-induced Smads were also associated with a significant increase in transcription activation as measured by luciferase reporter gene activity of Smad-dependent promoters. Inhibition of Smads by a dominant-negative mutant Smad4 in the DFMO-treated cells prevented the increased Smad transcription activation. Polyamine-deficient cells highly expressed TGF-beta and were growth-arrested at the G1 phase. Inhibition of TGF-beta by treatment with either immunoneutralizing anti-TGF-beta antibody or TGF-beta antisense oligodeoxyribonucleotides not only blocked the induction of Smad activity but also decreased the Smad-mediated transcriptional activation in polyamine-depleted cells. These findings suggest that Smads are involved in the downstream cellular processes mediated by cellular polyamines and that increased TGF-beta/TGF-beta receptor signaling following polyamine depletion activates Smads, thus resulting in the stimulation of Smad target gene expression.

Transforming growth factor-beta inhibits adipocyte differentiation by Smad3 interacting with CCAAT/enhancer-binding protein (C/EBP) and repressing C/EBP transactivation function

Transforming growth factor (TGF)-beta is a potent inhibitor of adipocyte differentiation. To identify which adipocyte transcription factors might be targeted by TGF-beta, we overexpressed key adipogenic transcription factors, C/EBPbeta, C/EBPdelta, or peroxisome proliferator-activated receptor (PPAR) gamma in NIH3T3 cells and tested the ability of TGF-beta to block adipogenesis. We show that TGF-beta inhibits adipocyte differentiation driven by either C/EBPbeta or C/EBPdelta without affecting C/EBP protein expression levels, suggesting that these C/EBPs are a direct target of TGF-beta action. Because TGF-beta inhibits adipogenesis by signaling through Smad3, we examined physical and functional interactions of Smad3 and Smad4 with C/EBPbeta, C/EBPdelta, and PPARgamma2. C/EBPbeta and C/EBPdelta were found to physically interact with Smad3 and Smad4, and Smad3 cooperated with Smad4 and TGF-beta signaling to repress the transcriptional activity of C/EBPs. Thus, repression of the activity of C/EBPs by Smad3/4 at C/EBP binding sites inhibited transcription from the PPARgamma2 and leptin promoters. In contrast, PPARgamma interacted only very weakly with Smad3 and its transcriptional activity was not repressed by Smad3/4 or in response to TGF-beta. Smad3/4 did not reduce the ability of C/EBP to bind to its cognate DNA sequence, but repressed transcription by inhibiting the transactivation function of C/EBP.

Differential modulation of androgen receptor-mediated transactivation by Smad3 and tumor suppressor Smad4

Smad proteins have been demonstrated to be key components in the transforming growth factor beta signaling cascade. Here we demonstrate that Smad4, together with Smad3, can interact with the androgen receptor (AR) in the DNA-binding and ligand-binding domains, which may result in the modulation of 5alpha-dihydrotestosterone-induced AR transactivation. Interestingly, in the prostate PC3 and LNCaP cells, addition of Smad3 can enhance AR transactivation, and co-transfection of Smad3 and Smad4 can then repress AR transactivation in various androgen response element-promoter reporter assays as well as Northern blot and reverse transcription-PCR quantitation assays with prostate-specific antigen mRNA expression. In contrast, in the SW480.C7 cells, lacking endogenous functional Smad4, the influence of Smad3 on AR transactivation is dependent on the various androgen response element-promoters. The influence of Smad3/Smad4 on the AR transactivation may involve the acetylation since the treatment of trichostatin A or sodium butyrate can reverse Smad3/Smad4-repressed AR transactivation and Smad3/Smad4 complex can also decrease the acetylation level of AR. Together, these results suggest that the interactions between AR, Smad3, and Smad4 may result in the differential regulation of the AR transactivation, which further strengthens their roles in the prostate cancer progression.

Repression of Smad2 and Smad3 transactivating activity by association with a novel splice variant of CCAAT-binding factor C subunit

Activation by transforming growth factor-beta (TGF-beta)/activin receptors leads to phosphorylation of Smad2 (Sma- and Mad-related protein 2) and Smad3, which function as transcription factors to regulate gene expression. Using the MH2 domain (Mad homologue domain of Smad proteins 2) of Smad3 in a yeast two-hybrid screening, we isolated a novel splice variant of CAATT-binding factor subunit C (CBF-C), designated CBF-Cb, that associated with Smad3. CBF-C is one of the subunits that form a heterotrimeric CBF complex capable of binding and activating the CAATT motif found in the promoters of many eukaryotic genes. CBF-Cb is 62 amino acids shorter than the wild-type CBF-C in the N-terminal region. In addition, CBF-Cb is expressed ubiquitously in various mouse tissues. By an immunoprecipitation assay, we detected an in vivo association of CBF-Cb with Smad2 and Smad3, independent of signalling by activated TGF-beta type I receptors. In transient transfection experiments, overexpression of CBF-Cb was able to repress the transactivating activity of Smad2 and Smad3, mediated either by direct binding to the Smad-responsive element or through their association with the Smad-interacting transcription factor FAST-2 (forkhead activin signal transducer-2). The Smad-mediated transcriptional response after TGF-beta receptor activation was also inhibited by overexpression of unspliced CBF-C. In addition, the repressive activity of CBF-Cb on Smad2- and Smad3-mediated transcriptional regulation was abrogated by co-expression of the general transcription activator p300. The association of CBF-Cb with Smad2 was competitively inhibited by overexpression of p300. These data indicate a novel mechanism for modulation of the transcriptional activity of Smad proteins, whereby the interaction of CBF-Cb, as well as canonical CBF-C, with the MH2 domain of Smads may prevent the association of Smads with transcriptional co-activators.

Regulation of cell proliferation by Smad proteins

Transforming growth factor-beta (TGF-beta) family members which include TGF-betas, activins, and bone morphogenetic proteins (BMPs) regulate a broad spectrum of biological responses on a large variety of cell types. TGF-beta family members initiate their cellular responses by binding to distinct receptors with intrinsic serine/threonine kinase activity and activation of specific downstream intracellular effectors termed Smad proteins. Smads relay the signal from the cell membrane to the nucleus, where they affect the transcription of target genes. Smad activation, subcellular distribution, and stability have been found to be intricately regulated and a broad array of transcription factors have been identified as Smad partners. Important activities of TGF-beta are its potent anti-mitogenic and pro-apoptotic effects that, at least in part, are mediated via Smad proteins. Escape from TGF-beta/Smad-induced growth inhibition and apoptosis is frequently observed in tumors. Certain Smads have been found to be mutated in specific types of cancer and gene ablation of particular Smads in mice has revealed increased rate of tumorigenesis. In late stage tumors, TGF-beta has been shown to function as a tumor promoter. TGF-beta can stimulate the de-differentiation of epithelial cells to malignant invasive and metastatic fibroblastic cells. Interestingly, TGF-beta may mediate these effects directly on tumor cells via subverted Smad-dependent and/or Smad-independent pathways.

Smad regulation in TGF-β signal transduction

Smad proteins transduce signals from transforming growth factor-β (TGF-β) superfamily ligands that regulate cell proliferation, differentiation and death through activation of receptor serine/threonine kinases. Phosphorylation of receptor-activated Smads (R-Smads) leads to formation of complexes with the common mediator Smad (Co-Smad), which are imported to the nucleus. Nuclear Smad oligomers bind to DNA and associate with transcription factors to regulate expression of target genes. Alternatively, nuclear R-Smads associate with ubiquitin ligases and promote degradation of transcriptional repressors, thus facilitating target gene regulation by TGF-β. Smads themselves can also become ubiquitinated and are degraded by proteasomes. Finally, the inhibitory Smads (I-Smads) block phosphorylation of R-Smads by the receptors and promote ubiquitination and degradation of receptor complexes, thus inhibiting signalling.